1. Field of the Invention
The present invention provides a high voltage spark gap switch controlled by a laser-photocathode subsystem. A double-triggering mechanism is used for reliably closing the spark gap switch while keeping the low jitter properties of the laser triggering.
2. Description of Prior Art
High voltage switch is one of the elementary devices employed in pulsed power techniques. Ideally, it controls the flow of current in a circuit in two states: either the current flows at a value determined by the other components in series with it, or the current does not flow at all. There are many considerations when choosing a switch; for example, large current and/or voltage handling capacity, compact size, price, durability and reliability. A switch with minimized delay and jitter is also needed in many applications.
A spark gap is one of the most widely used switches. It is relatively simple to build and easy to operate and its application range is flexible. It can conduct current from a few tens of amperes to multi-mega amperes and it can also withstand voltages up to several megavolts. The basic spark gap usually consists of two current-carrying electrodes separated by a gap filled with isolating medium, which is made to break down by overvolting the gap or by some other means such as applying a triggering pulse through a third electrode, i.e. a trigger electrode, injecting an electron beam or shining an optical beam into the gap.
Different methods of inducing electrical breakdown in the spark gap have their own advantages and drawbacks. For example, the electrical pulse triggered switch is comparably simple, but it needs a separate electrical pulse generator, and the delay and jitter of the breakdown induced by this method are relatively larger. For precise timing and synchronization, the laser-triggered switch has been extensively studied in the past several decades. Though great progress has been made, some problems remain for the laser-triggered switches. One of them is how to make use of laser optical energy efficiently because many mediums used in the spark gap are transparent and their absorption coefficients are rather low to the photons generated by a common laser source whose wavelength is in ultra-violet (UV) spectrum or longer. For example, SF6 and N2 are two of the most often used gaseous mediums in spark gap switches. But neither SF6 nor N2 under standard conditions has an absorption coefficient higher than 0.002/cm for the photons with a wavelength of 186 nm. It means that the ratio of the photons absorbed by the gases to the total photons in the laser beam is less than 6% after the laser beam with the wavelength passes through a 30-cm-long gas channel. Moreover, the number of absorbed photons tends to decrease with the increment of the photon wavelength in UV spectrum. Therefore, most of photons in the laser beam passing through a common gas gap whose length is generally less than 30 cm simply waste their optical energy. For this reason, a high-energy laser system is needed to trigger a traditional high voltage spark gap switch, thus incurring a very high cost for the laser system. Otherwise, the delay time and the jitter of the switch will be adversely impacted.
One of the purposes of this invention is to seek a viable method to convert the leftover optical energy of the laser beam to actual triggering energy in order that the laser energy can be utilized efficiently. In the mean time, depressing the jitter of a spark gap switch as well as closing it reliably is the concern of current invention, too. Further objects and advantages of the invention will become apparent from a consideration of the drawings and ensuing description.
U.S. Pat. No. 5,057,740 to George et al. describes the use of photoelectrons to trigger a backlight thyratron switch. The switch exposes photoemission materials having very low quantum efficiency directly to a low-pressure gas. The working range of their switch is limited to only the left hand side of the Paschen curve. The lifetimes of the photoemission materials are also limited. Furthermore, the medium in such switches can only be gases.